1. Field of the Invention
This invention relates to a method of adjusting a head gap for a wire dot impact printer and, more particularly, to a method of adjusting a head gap capable of automatically obtaining an optimum value of the gap corresponding to the characteristics of a wire dot head.
2. Description of Related Art
In a conventional wire dot impact printer, a wire dot head is disposed so as to oppose a platen through an ink ribbon and printing media, and prints onto the printing media by impacting the ink ribbon with printing wires. Such a wire dot impact printer of this kind can use printing media of various types and can adjust the distance between the tip of the wire dot head and the printing media, or a head gap, to an optimum value when the thickness of the printing media or a number of sheets, in the case of copy paper, is changed due to changes of the printing media or the like.
FIG. 35 is a flow chart showing a conventional method of adjusting a head gap for a wire dot impact printer; FIG. 36 is a diagram showing a printing sample of the conventional method of adjusting the head gap for the wire dot impact printer. In FIG. 36, (a) is a diagram showing printing data in a first line of printing; (b) is a diagram showing a printing pattern of test printing; (c) is a diagram showing a printing pattern of re-printing.
Referring to FIG. 35, the power of the wire dot impact printer is turned on at step S1. At step S2, a judgment is made as to whether or not there are printing media, and if there are, the program goes to step S3, or if there are not, the program waits for the media. Then, printing data from a host computer (not shown) is received at step S3. The position of the wire head is set so that the head gap g becomes a reference head gap gA (for instance 0.5 mm) for test printing at step S4. Herein, the reference head gap gA is defined as a head gap g under a condition that an ink ribbon (not shown) and printing media P, whose thickness is previously known, are set, and a standard printing time T.sub.S is previously written in a table in a ROM. According to the printing data received, at step S5 a test printing, e.g., the printing of several dots to several tens of dots of a first printing line, is performed as shown in FIG. 36(b), and during the test printing, printing time T is detected. At step S6, the difference between the detected printing time T and the standard printing time Ts stored in the ROM is calculated. A difference Dg between the standard head gap gA and an actual head gap g is then calculated using the relationship where a difference of 3 .mu.sec in the printing time T corresponds to a head gap g of 0.01 mm. The program step calculates based on the thickness of the printing media P at the time when the standard printing time Ts is determined, and the difference .DELTA.g, thereby finding the thickness of the printing media currently set. At step S7, the program step calculates a shifting amount of the wire dot head to shift the head gap g to an optimum value gR corresponding to the thickness of the printing media P, and automatically adjusts the head gap with driving means for changing gap. Then, an actual printing is performed for the line on which the test printing is done, as shown in FIG. 36(c), at step S8. Hereafter, an ordinary printing is done at step S9.
In the conventional method of adjusting the head gap for the wire dot impact printer as described above, however, it is difficult to adjust the head gap g accurately since there are deviations, by each wire dot head, in the standard printing time Ts in the ROM used for calculating the head gap g, and in the detected printing time T.